EFFECT OF THE METAL-SUPPORT INTERACTION ON THE CATALYTIC PROPERTIES OF PALLADIUM FOR THE CONVERSION OF DIFLUORODICHLOROMETHANE WITH HYDROGEN - COMPARISON OF OXIDES AND FLUORIDES AS SUPPORTS

The reaction of difluorodichloromethane with hydrogen has been studied between 433 and 523 K and atmospheric pressure, over Pd catalysts supported on graphite and oxides or fluorides of Al, Ti, and Zr. In CF2Cl2 hydrogenation, CH2F2 and CH4 represented more than 95% of the products. The catalytic properties of fluoride supported catalytsts did not undergo any change as a function of time. In contrast, Pd supported on oxides showed changes in selectivty during the first hours on stream. This was ascribed to the reaction of the oxide support with HF released during the reaction. Alumina and titania were nearly completely converted to the corresponding fluorides, but not zirconia. The selectivity to the desired product CH2F2 was 56% for Pd/graphite and reached 90% for Pd/ZrF4. The kinetic study suggested that the selectivity was controlled by the bond strength between a carbene-like species CF2 and the surface. The strength of this interaction is supposed to vary with electron availability at the Pd surface, and this hypothesis was then investigated by infrared spectroscopy using the adsorption of CO on Pd/Al2O3 and Pd/AlF3. The results show that the morphology of the Pd particles was little affected by the support and that AlF3-supported Pd becomes electron deficient, due to the strong Lewis acidity of the support. This effect is mainly a short-range effect which is better induced by supports made up of a mixture of fluorides, oxyfluorides, and hydroxyfluorides, rather than pure fluorides. Catalytic properties similar to those of Pd/AlF3 and ZrF4 can be simulated with Pd/graphite samples promoted with small amounts of aluminum or zirconium.